As electronic devices increasingly become portable, advances must be made in energy storage devices so as to enable such portability. Indeed, it is often the case with current electronics technology that the limiting factor to portability of a given device is the same size and weight of the associated energy storage device. Obviously a small energy storage device may be fabricated for a given electrical device, but at the cost of energy capacity. The result is either that the energy source is too bulky, too heavy, or does not last long enough for a given application. The main energy storage device used for portable electronics today is the electrochemical battery cell, and increasingly the electrochemical capacitor.
Electrochemical capacitors are a class of devices characterized by relatively high power densities as compared with conventional battery systems. The charge mechanism of electrochemical capacitors is typically the result of primary, secondary, tertiary and higher order oxidation/reduction reactions between the electrodes and the electrolyte of the capacitor.
Heretofore, such devices have typically been made of electrodes fabricated of relatively exotic or expensive materials such as ruthenium. Electronically conducting polymers represent a promising class of materials for the development of electrochemical capacitors; common examples of such polymers include polypyrrole and polyaniline. High specific capacitance may be obtained in these materials by doping the polymer via an oxidation/reduction reaction. The simplest such devices are symmetric capacitors in which both the anode and cathode are fabricated of the same conducting polymer. Devices with different or asymmetric electrodes are also possible.
One of the major limitations to the economical fabrication of polymer electrochemical capacitor devices is the requirement that such devices be fabricated with noble metal substrates upon which the polymer electrodes are deposited. Such noble metal substrates are extremely costly, and thus, substantially increase the expense of fabricating polymer electrodes for electrochemical cells. Noble metals are necessary as the substrates upon which to deposit polymers since only they provide the adherence between the polymer and the substrate necessary in order to provide a device with long cycle life.
Carbon based materials have also been used as substrates for the deposition of electrically conducted polymers for electrochemical capacitor devices. Carbon, while cheaper than noble metals, such as gold and platinum, results in higher equivalent series resistance ("ESR") than in noble metal substrates. High ESR yields an energy storage device which cannot provide the high energy and power levels required of most capacitor devices.
Accordingly, a method by which the use of non-noble metals and non-noble metal alloy foils would be preferred to simplify the manufacturing processes, particularly as compared to the carbon electrodes. This will allow fabrication costs to be reduced substantially, as well as reducing ESR. Direct electrodeposition of polymer materials onto non-noble metal and metal alloy surfaces would also decrease manufacturing steps and costs, while yielding devices which can provide the performance required.